The OpenBSD guide says nothing about CHS mode so I'm not sure how to do this. I just selected 'no' so now I need the offset of my OpenBSD (A6) partition (e 1) so I'm stuck between 312581807 + 1 or 312581807 + 63. Or is it something else?

Okay I just noticed something doesn't look right. It seems as if the total disk sectors should equal to the size of all the partitions (if all the space is used) but it looks like fdisk sees the EFI GPT partition was the entire disk for some reason? Even though it's only 200-something megabytes.

Wait I'm going to try and fix this; I think the partition is indeed the entire disk so I'm gonna free up some space with GParted to make some unallocated

For ease of any computation, and avoiding having to type giant numbers, just use CHS mode, and select your starting point to be cylinder 19458; the prompts will guide you to the ranges for head/sector for the starting point, and for then end point, will show the maximums.

But you have a different problem. Your disk is already full. Partition 0 consumes the whole thing.

I don't often set up disks for multiboot, or sharing between OSes. And I don't have a test system handy at this instant to give you any more detailed guidance once you "shrink" your existing partition. But just go by the output of an interactive fdisk session in CHS mode, and you should be fine. Before writing out your MBR, double check that you don't have any overlays.